Conventionally, noise reduction of motors has been attempted, for example, by reducing the operating noise of motors or shifting such operating noise out of the audible frequency range. Generally, motors are designed in an optimal manner to reduce noise by selecting the materials and shapes of components that affect the resonant frequencies of the components and the number of poles of the motor. However, it is difficult and troublesome to reduce noise of motors by optimizing only the mechanical features of the components.
For example, Japanese Laid-Open Patent Publication No. 2007-1366 discloses a technology for reducing the operating noise of a motor through electric control, namely, by adjusting the direct-current voltage supplied to the motor.
Including the technology disclosed in Japanese Laid-Open Patent Publication No. 2007-1366, technologies for reducing motor operating noise have been widely known. In contrast, the present inventors are considering improving the sound quality of motor operating noise by adjusting the tone of motor operating noise, instead of attempting to reduce the operating noise. Although electric control is an easy and preferable way to improve the sound quality of motor operating noise, more consideration is needed on what kind of control would be the most appropriate.